1. Field of the Invention
The present invention relates to a method for producing an optical fiber having low polarization mode dispersion.
2. Description of the Related Art
Optical signals transmitted through single-mode optical fibers comprise two orthogonal polarization modes that, in case of a fiber with a perfectly cylindrical core of uniform diameter, propagate at a common velocity. However, in real optical fibers the core cylindrical symmetry may be disrupted due to shape defects or non-uniform stresses. As a result, a phase difference can accumulate between the two modes, and the fiber is said to show “birefringence”. In particular, the birefringence introduced by shape and internal stress asymmetry is known as “intrinsic linear birefringence”.
The structural and geometrical irregularities of the optical fiber that give rise to birefringence typically originate from the preform itself and are modified during the process of drawing the fiber. This drawing process is usually carried out by means of an apparatus known as a “drawing tower”, starting from a glass preform. In practice, after the preform has been placed in a vertical position and heated to a temperature above the softening point within a suitable furnace, the molten material is drawn downwards at a controlled speed in such a way as to produce a threadlike element that forms the optical fiber itself. In this process, asymmetrical stresses are typically introduced in the fiber.
In a birefringent fiber, the two components of the fundamental mode, initially in phase with each other, come to be in phase again only after a certain length, commonly known as the “beat length”. In other words, the beat length is the period of repetition of a certain state of polarization (on the assumption that the fiber maintains a constant birefringence over this length).
In the so-called “polarization-preserving fibers”, asymmetry is deliberately introduced into the fiber to generate birefringence. However, in ordinary (i.e. non-polarization-preserving) fibers, birefringence is detrimental. In fact, when pulsed signals are transmitted into an optical fiber, the birefringence is a potential cause of pulse spreading, since the two polarization components excited by the pulses travel at different group velocities (i.e. become dispersed). This phenomenon, known as polarization mode dispersion (PMD), has been widely studied in recent years because of its importance in periodically amplified light guide systems.
Typically, the phenomenon of PMD leads to a limitation of the width of the signal transmission band and, consequently, a degradation of the performance of the optical fibers along which the aforesaid signals are transmitted. This phenomenon is therefore undesirable in systems of signal transmission along optical fibers, especially in those operating over long distances, in which it is necessary to minimize any form of attenuation or dispersion of the signals to guarantee high performances in transmission and reception.
In order to reduce the polarization mode dispersion in an optical fiber it has been proposed to spin the fiber during the drawing process so that the fiber is caused to twist around its longitudinal axis, with a resulting torsional deformation of the viscous zone of the fiber material in the furnace; this deformation is frozen into the fiber as the fiber looses its viscous status by cooling.
Due to spinning, a rotation of the polarisation axes of the fiber is impressed on (and frozen into) the fiber. As a result, when the optical pulses are transmitted into the optical fiber, they propagate alternately on the slow and fast birefringence axes, thus compensating the relative delay and reducing the pulse spreading. This is equivalent to have a local effective refractive index for the pulses equal to the mean refractive index on the two axes, the average being taken over the pulse length along the fiber.
In the present description and claims                with “applied spin” or “applied torsion” it is intended the torsion applied to the fiber during the drawing process by a spinning apparatus so that the fiber is caused to rotate around its longitudinal axis;        with “actually applied spin” or “actually applied torsion” it is intended the torsion effectively applied to the fiber during the drawing process notwithstanding possible mechanical effects, e.g., slippage, at the interface between the fiber and the spinning apparatus;        with “viscous zone” is intended a longitudinal portion of the glass material of the optical fiber that in the furnace has a temperature sufficiently high to be in a viscous status;        with “viscous zone length” is intended the length of said longitudinal portion in a viscous status, which substantially corresponds to the length of the portion of the furnace comprised between its hottest point and its exit point; for example, in a furnace comprising an upper muffle, a core muffle defining a hot zone, and a lower muffle, the viscous zone length may be approximated with the length of the portion of the furnace comprised between the central part of the hot zone and the lower end of the lower muffle;        with “frozen-in spin” or “frozen-in torsion” it is intended the torsion permanently impressed on the fiber, when cooled, during the spinning process as a result of the torsional deformation undergone by the viscous zone of the fiber material in the furnace;        with “maximum applied spin” or “maximum applied torsion” is intended the maximum value of the applied torsion;        with “maximum frozen-in spin” or “maximum frozen-in torsion” is intended the maximum value of the frozen-in torsion;        with “detorsion” is intended a torsion having a direction opposite to the direction of a previous torsion;        with “at least a 50% detorsion” is intended a detorsion adapted to impart an angular displacement equal to at least half the angular displacement imparted by the previous torsion;        with “recovery” is intended the ratio (Tappl−Tfr)/Tappl, wherein Tappl is the maximum actually applied torsion and Tfr is the maximum frozen-in torsion;        with “substantially sinusoidal spin” is intended, as disclosed by the document U.S. Pat. No. 6,240,748, a spin function wherein the magnitude of the coefficient for one of its oscillatory components (the fundamental component) dominates the magnitude of the coefficients for all other oscillatory components (the secondary components) as well as the coefficient for any constant component. In quantitative terms, domination occurs when the magnitude of the coefficient for the fundamental component is at least about three times the magnitude of the coefficient for each of the secondary components and the coefficient of the constant component. The magnitude of said coefficients can be determined by performing a complex Fourier analysis of the spin function using conventional techniques well known in the art.        
U.S. Pat. No. 5,298,047 discloses that PMD can be substantially reduced if, during drawing of the fiber, a torsion is applied to the fiber such that a permanent spin (i.e., a permanent torsional deformation) is impressed on the fiber. The torsion is applied such that the spin impressed on the fiber has alternately clockwise and counterclockwise helicity. This document states that fibers having impressed spin lower than 4 spins/meter do not exhibit commercially significant reduction in PMD. Thus, it teaches to apply a torsion to the fiber such that the spin impressed on (frozen into) the fiber is, in at least a portion thereof, in excess of 4 spins/meter, preferably in excess of 10 or even 20 spins/meter.
U.S. Pat. No. 6,240,748 states that a conventional sinusoidal spin function, as that disclosed by the above mentioned U.S. Pat. No. 5,298,047 document, is capable of reducing PMD only for a small number of fiber beat lengths. For example, U.S. Pat. No. 6,240,748 shows that a conventional sinusoidal spin function having a spin amplitude of 3 turns/meter and a spatial frequency f of 2 meter−1, while obtaining a low PMD reduction factor at a beat length of approximately a quarter of a meter, does not maintain this PMD reduction for longer beat lengths. Therefore, U.S. Pat. No. 6,240,748 teaches to reduce PMD in single mode optical fiber by spinning the fiber during the drawing process in accordance with a spin function having sufficient harmonic content to achieve low level of PMD for commercial fibers for which the beat lengths of the fibers, including the beat lengths of different portions of the fibers, are variable and thus not readily known in advance. Examples of suitable spin functions capable of achieving low levels of PMD for a variety of beat lengths include frequency-modulated and amplitude-modulated sine waves. However, the Applicant notes that the spin functions disclosed by this latter document require the use of a complex and expensive spinning apparatus.
Moreover, U.S. Pat. No 6,240,748 states that the spin function employed in producing a fiber and the resulting (frozen-in) spin function present in the finished fiber are not in general identical because of mechanical effects, e.g., slippage, at the interface between the fiber and the apparatus used to apply the spin function to the fiber. However, it states that the correspondence between the spin function applied by the spinning apparatus and the resulting spin function in the fiber is in general good enough to achieve the benefits of the invention disclosed therein.